Add UMA to log the size of the Previews opt out DB

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 0fdeeb1de72d951a07241bc23f21cb28593dbcd5..7e1cbfe13a01f7ef810f16455e4057b58e04bfd5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -47951,20 +47951,27 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>
     Samples of user interactions with the previews Offline &quot;Faster page
     loaded&quot; infobar. These samples include:
 
     Displays of the infobar and clicks on the &quot;Load original&quot; link.
     Whether the infobar was dismissed by navigation or the user clicking on
     &quot;X&quot; close button.
   </summary>
 </histogram>
 
+<histogram name="Previews.OptOut.DBRowCount">

[Alexei Svitkine (slow), 2016/10/26 15:16:12]

Nit: add units="rows"

[RyanSturm, 2016/10/26 18:16:29]

Done.

+  <owner>ryansturm@chromium.org</owner>
+  <summary>
+    The number of rows in the Previews opt out SQLite table at profile startup.
+  </summary>
+</histogram>
+
 <histogram name="Previews.OriginalContentLength" units="KB">
   <owner>ryansturm@chromium.org</owner>
   <summary>
     The estimated content length of a page load using a preview optimization.
   </summary>
 </histogram>
 
 <histogram name="PrinterService.PrinterServiceEvent"
     enum="PrinterServiceEventType">
   <owner>vitalybuka@chromium.org</owner>